Crib Sizes and Mattress Fit, Explained
A full-size crib takes a standard mattress at least 27 1/4 by 51 5/8 inches, a mini crib takes its own smaller one, and the mattress drops before baby can sit.
By Amanda RoseContributing writer.
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The short answer
In the US, a crib is either full-size or non-full-size. Full-size cribs all share one standard interior, so any standard crib mattress fits them. Mini cribs, play yards and other non-full-size cribs come in many sizes, and each needs a mattress made for that model.
Whatever the crib, the fit test is the same: the American Academy of Pediatrics (AAP) says that if you can push more than two fingers between the mattress and the side or end of the crib, do not use that crib and mattress together.
Standard crib size
Federal rules define the size by the inside of the crib. A crib counts as non-full-size if its interior is longer than 55 inches or shorter than 49 3/4 inches, or wider than 30 5/8 inches or narrower than 25 3/8 inches. Full-size cribs sit inside that window.
For the mattress, the federal crib mattress standard (16 CFR part 1241) says a full-size crib mattress must measure at least 27 1/4 inches wide and 51 5/8 inches long. That minimum is what keeps the gaps around the edge small. Other AAP checks on its crib guide:
- Slats no more than 2 3/8 inches apart.
- Rails at least 26 inches above the mattress support at its lowest setting.
- A firm mattress that does not sag under the baby.
- A crib made after June 2011, when the current federal standards (including the end of drop-side cribs) took effect.
Mini crib vs standard crib
| Mini crib | Standard (full-size) crib | |
|---|---|---|
| Federal category | Non-full-size | Full-size |
| Mattress | Made for that model or size | Any standard crib mattress |
| Room needed | Less | More |
| How long it lasts | Until the child outgrows the smaller frame | Until about 35 inches tall; many convert to a toddler bed |
A mini crib suits a small room or a parents' bedroom. Because sizes vary, buy the mattress the crib maker specifies, or one sold for that exact interior size, and do the two-finger test. Of the 292 cribs on Wiki Shopping on October 26, 2026, 56 are listed as full-size and 30 as mini; the rest do not say.
When to lower the crib mattress
Crib mattresses sit on an adjustable support. The AAP's room safety checklist gives the order:
- Before the baby can sit: lower it to where the baby cannot fall out by leaning or pulling over the rail.
- Before the baby can stand: set it to the lowest position.
- Move to a bed at 35 inches tall, or when the side rail is less than three-quarters of the child's height, roughly at nipple level.
The AAP notes that falls are the most common crib injury, usually from a mattress left too high for the child's size.
Convertible cribs
Many full-size cribs convert into a toddler bed, a daybed or a full-size bed frame, sold as 3-in-1, 4-in-1 and up. On Wiki Shopping, 131 cribs are 4-in-1, the largest group. Check the listing for whether the conversion parts are included or sold separately.
How we count
Crib size and conversion stages are read from each crib's store listing. Cribs that do not state them are left out of those counts, and the counts change as listings are added or removed.
Shop by spec
Cribs
3 Converts (N-in-1)
- MID-CENTURY MODERN STYLE: The Scoot's angular feet and contrast horizontal lines create a charming retro impression that is current and on-trend. Complete your nursery with the Scoot 3-drawer dresser.
- GREENGUARD Gold Certified: The Scoot has undergone rigorous scientific testing for over 10,000 chemical emissions and VOCs. It contributes to cleaner indoor air, creating a healthier environment for your baby to sleep, play, and grow.
- 3-IN-1 CONVERTABILITY & ADJUSTABLE HEIGHT: The Scoot transitions from crib to toddler bed and daybed, with an included toddler bed conversion kit. The low profile design and 4 adjustable mattress positions make it easy to put a newborn to rest.
- SAFE FOR BABE: We go the extra step to ensure your babe's safety. Finished in a non-toxic multi-step painting and staining process that is lead and phthalate safe which exceeds ASTM International and U.S. CPSC safety standards.
- SUSTAINABLE DESIGN: The frame and spindles on the Scoot are made with 100% sustainably sourced solid New Zealand pine wood, one of the most renewable woods available. Our hidden hardware ensures a clean look with no visible screws or bolts seen on the furniture exterior.
- Converts to a reading nook (cushions sold separately)
- Converts to a daybed and toddler bed (rails included)
- Solid ash wood frame and slats with ash-veneered MDF, birch plywood
- Solid aluminum feet with brass electroplating
2 Converts (N-in-1), Full-size
- STYLE & FUNCTION: The Maki is the modern lifestyle essential with its clean design, removable wheels, and foldable lightweight frame for easy storage.
- GREENGUARD GOLD CERTIFIED: The Maki has undergone rigorous scientific testing for over 10,000 chemical emissions and VOCs. It contributes to cleaner indoor air, creating a healthier environment for your baby to sleep, play, and grow.
- 2-IN-1 CONVERTABILITY & ADJUSTABLE HEIGHT: The Maki transitions from crib to toddler bed, with an included toddler bed conversion kit. The low profile design and 4 adjustable mattress positions make it easy to put a newborn to rest.
- SUSTAINABLE DESIGN: Made with sustainably sourced solid New Zealand pine wood, one of the most renewable woods available. The mattress support is TSCA Compliant engineered wood.
- SAFE FOR BABE: We go the extra step to ensure your babe's safety. Finished in a non-toxic multi-step painting and staining process that is lead and phthalate safe.
3 Converts (N-in-1), Mini
- URBAN STYLE AND SIZE: Designed for urban living with a foldable frame and removable, locking caster wheels for easy storage and transport; Suitable for a growing babe from newborn to beyond 18 months; Safe to use until 35" tall or starting to climb.
- GREENGUARD GOLD CERTIFIED: The Origami has undergone rigorous scientific testing for over 10,000 chemical emissions and VOCs; It contributes to cleaner indoor air, creating a healthier environment for your baby to sleep, play, and grow
- ADJUSTABLE MATTRESS HEIGHT: Low-profile design and 2 adjustable mattress positions make it easy to put a newborn to rest; Standard mini size crib which includes 1" mattress pad; Pure by Babyletto mini crib mattress (37" x 23.5") is recommended.
- FSC CERTIFIED WOOD: Wood used in this crib comes from well-managed, FSC-certified forests. The FSC label means that wood has been harvested to benefit communities, wildlife, and the environment. The mattress support is TSCA compliant engineered wood.
- SAFE FOR BABE: Finished in a non-toxic multi-step painting and staining process that is lead and phthalate safe.